such as "Introduction", "Conclusion"..etc
Some scientific results are hard to spot, especially in genetic
research. Often scientists are unable to physically see if the gene they
inserted into a cell has produced the desired trait. To overcome this
problem researchers use various genetic markers that contain pieces of
foreign DNA that cause cells to, for example, glow when exposed to
But scientists in the lab of Whitehead Member Rudolf Jaenisch didn't
have to resort to these genetic markers in their latest experiment
because the results were easy to see. Building on their widely
publicized June Nature paper, which demonstrated that it's possible to
convert specialized mouse skin cells into unspecialized stem cells,
Whitehead postdoctoral researchers Alexander Meissner and Marius Wernig
have now identified successfully reprogrammed cells by looks alone.
Their findings, which appear online in the journal Nature
Biotechnology, bring human stem cell therapies a step closer to reality.
Before reprogramming can be applied to our own species to generate
custom embryonic stem cells, scientists must be able to accomplish it
without altering the DNA of the cells involved.
"This eliminates one of the major hurdles to reprogramming human
cells," says Jaenisch, who is also an MIT professor of biology. "If we
overcome the other obstacles, this approach could one day provide custom
human embryonic stem cells for use in therapy."
Last spring, Wernig and Meissner relied on genetic markers to
identify successfully reprogrammed cells. This required them to work
with fibroblasts from a genetically modified mouse. The mouse was grown
from embryonic stem cells that contained foreign DNA coding for
antibiotic resistance. The scientists had strategically inserted these
foreign DNA "markers" at particular points along the genome, next to
genes expressed only in embryonic stem cells. All of the cells
(including fibroblasts) in the resulting mouse contained the markers.
In the original experiment, the researchers took fibroblasts from the
tail of this mouse and infected them with a special virus containing
four genes (Oct4, Sox2, c-myc, and Klf4) capable of converting the cells
to an embryonic state. Genes typically active in embryonic stem cells
roared to life, triggering the adjacent foreign DNA to provide
antibiotic resistance. Thus only fully reprogrammed cells survived
exposure to an antibiotic, which allowed the scientists to isolate them.
"When we conducted the original experiment, we noticed that many of
the infected cells had already started to change shape before the
markers were activated," says Wernig.
So they set up a new experiment to test if visual identification
alone would work. Indeed, they were able to separate the reprogrammed
cells from ordinary fibroblasts under a microscope, based on several
physical differences. Fibroblasts are big and flat. Embryonic stem
cells are small, round and form tight colonies.
"We've shown that there's no need to use markers to isolate
successfully reprogrammed cells," says Meissner. "This significantly
simplifies this approach in mice, as we can now work with ordinary
But another hurdle remains before the technique can be applied to
"We still used viruses containing foreign DNA to introduce the genes
that induced the reprogramming," explains Meissner.
The scientists are now working to eliminate the virus from the
reprogramming process. Jaenisch believes they will eventually succeed
and points out that the technique could eventually yield a bountiful
supply of custom human embryonic stem cells for use in therapy.
Meissner and Wernig successfully reprogrammed about 0.5 percent of
the fibroblasts. Given that there are millions of cells in a typical
skin biopsy (researchers used skin from either the end of the tail or
from the ear of the mouse), that translates into thousands of stem
cells, each one capable of developing into any cell type of the body.
Reference: Nature Biotechnology, online edition, August 27, 2007,
"Direct Reprogramming of Genetically Unmodified Fibroblasts into
Pluripotent Stem Cells" Alexander Meissner(1*), Marius Wernig(1*) and
*These authors contributed equally to this work.
(1) Whitehead Institute for Biomedical Research, Cambridge,
Massachusetts (2) Department of Biology, Massachusetts Institute of
Technology, Cambridge, Massachusetts
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